This section provides background information related to the present disclosure which is not necessarily prior art.
Lightweight metal components have become ever more prevalent when manufacturing vehicles, especially automobiles, where continual improvement in fuel efficiency and performance is desirable. Lightweight metal components for such automotive applications are often made of aluminum, magnesium, and alloys thereof. Such lightweight metals can form components that may be load bearing and need to be strong and stiff, having good strength and ductility (e.g., elongation). High strength and ductility are particularly important for safety requirements and durability in vehicles like automobiles.
An exemplary lightweight metal alloy for structural components in a vehicle is an aluminum-containing alloy. Aluminum-containing alloys can be formed by wrought processes, such as extrusion, forging, stamping, or a casting technique, such as die-casting, sand casting, permanent-mold casting, investment casting, and the like. In such casting, a molten metal may be poured into a casting mold. The molten metal conforms to a shape within the casting mold and thus adopts the shape of the mold cavity as it cools and solidifies. After the metal is solidified and forms a part, the mold is then separated and removed from the part. In a die-casting process, the molten metal material passes through a die defining one or more orifices or apertures, often under pressure. After passing through the in-gate, runners and gating in the die, the molten metal enters a mold cavity where it solidifies to complete the casting process.
All die castings have a very thin chill zone on the outer surfaces of the cast part, which occurs adjacent to the cooler walls of the mold. The chill zone has a different microstructure than other regions of the part. The chill zone is adjacent to an internal dendritic microstructure region that extends from the chill zone towards an interior or center of the cast part. The chill zone is typically only a very small percentage of the total thickness of the part.
When casting alloys, industry standards and limitations during the casting process typically determine which alloy materials and alloying constituents are included. Alloy selection is ultimately tailored to the dendritic microstructure region properties that are needed for the part, while the chill zone microstructure is usually ignored. Sometimes, the chill zone may be partially removed after casting to meet surface roughness, surface appearance, and/or assembly requirements. Strength and other alloy properties could be further improved in view of these conventional casting techniques. Lightweight metal castings, such as aluminum and magnesium castings, need higher strength levels commensurate with those of high strength wrought aluminum and steel stampings. Thus, there is an ongoing need for improved casting processes to form improved lightweight metal components from alloys having suitable castability, strength, and ductility among other characteristics.